Piston with undulated skirt



y 16, 1967 w. P. HOLCOMBE 3,319,535

I PISTON WITH UNDULATED SKIRT Filed Aug. 19, 1965 5 Sheets-Sheet 1 x w ATTORNEYS y 6, 1967 w. P. HOLCOMBE PISTON WITH UNDULATED SKIRT 5 Sheets-Sheet 2 Filed Aug. 19, 1965 I N VEN TOR. W444 466' fl m /P 6 0: (OM56 ATTORNEYS I y 6, 1967 w. P. HOLCOMBE 3,319,535

PISTON WITH UNDULATED SKIRT Filed Aug. 19. 1965 5 Sheets-Sheet 5 I NVEN TOR. M/zJLL flCE fi m/P A OL coM56 w ATTORNEYS m vqw w United States Patent 3,319,535 PISTON WITH UNDULATED SKIRT Wallace Philip Holcombe, Birmingham, Mic-11., assignor to TRW Inc., Cleveland, Ohio, a corporation of Ohio Filed Aug. 19, 1965, Ser. No. 480,845 8 Claims. (Cl. 92-408) This invention relates to lightweight, strong and quiet operating engine pistons and more specifically relates to pistons with undulated skirts which have enhanced flexibility and minimum audible piston slap.

The invention will hereinafter be described as specifically embodied in extruded aluminum internal combustion engine pistons, but it should be understood that the principles of this invention are generally applicable to pistons of any suitable metal made by any suitable process, including casting.

The conventional engine pistons have skirts at the outboard ends of the pin bosses. These skirts take the form of complete cylinders or cylinders with flat sides at the outboard ends of the piston bosses. Such pistons, when subjected to side load normal to the pin boss axis, and when heated in engine operation, are sufliciently flexible to be hoop stretched thereby counteracting expansion and maintaining relatively quiet operation. However, in large bore diameter high performance engines, the exten sive skirt area confronting the cylinder provided by such full skirt or substantially full skirt pistons, is not desired. Therefore, the so-called racing pistons have been provided with skirts which terminate at the inboard ends of the piston bosses, and the skirt areas confronting the cylinder normal to the pin boss axis, are quite narrow. These pistons are compact, lightweight, strong and have a low friction factor, but they are much too stiff for quiet operation and their use has been confined to racing engines where noise is not a factor.

This invention is more applicable to extruded pistons due to the higher elongation of the metal from the extrusion process which allows thinner, more flexible section to be used.

The present invention now provides pistons with skirt walls at the inboard ends of the pin bosses as in the socalled racing pistons, but quiet operation is achieved by undulating the skirt walls from an innermost position at the inboard ends of the pin bosses to an outermost position which is aligned with the axial middle of the pin bosses. This undulation increases the flexibility of the cylinder wall engaging skirt portions, thereby providing quiet operation without increasing operating friction to the degree involvedwithpistons having skirts extended to the outboard ends of the pin bosses. The undulated skirt has an eifective beam length much greater than the racing piston which has its cylinder engaging skirt portions confined between the pin bosses and joined to the inboard ends of the pin bosses along fiat Walls.

It is then an object of this invention to provide high performance, low friction engine pistons having suflicient skirt flexibility to warrant quiet operation.

Another object of the invention is to provide so-called racing pistons with flexible skirts to decrease audible piston slap in the operation of such pistons.

Another object of the invention is to provide quiet operating, low friction engine pistons which are sufficiently flexible so as not to require close piston-to- .cylinder fits and which are sufficiently stifi to withstand collapse in high performance engines.

Another object of the invention is to provide engine pistons with undulated skirts having flat sides at the inboard ends of the piston bosses and curling outwardly to cylinder wall engaging portions normal to the pin boss axis.

Another object of the invention is to increase the beam 3,319,535 Patented May 16, 1967 length of cylinder wall engaging skirt portions of engine pistons while providing flat skirt walls at the inboard ends of the piston pin bosses.

A specific object of the invention is to provide an engine piston with an undulated skirt having thin areas at the centers of the thrust faces and increasing in thickness from these centers to flat wall sections at the inboard ends of the pin bosses.

Other and further objects of this invention will be apparent to those skilled in this art from the following detailed descriptions of the annexed sheets of drawings which, by way of preferred examples, illustrate several embodiments of the invention and also compare the in- Vention with the prior art.

On the drawings:

FIGURE 1 is a side elevational view of one form of engine piston according to this invention;

FIGURE 2 is a bottom plan view of the piston of FIGURE 1;

FIGURE 3 is a transverse cross sectional view of the piston of FIGURES 1 and 2 taken along the line III-III of FIGURE 2;

FIGURE 4 is a side elevational view of a modified piston according to this invention;

FIGURE 5 is a transverse cross sectional view taken along the line VV of FIGURE 4;

FIGURE 6 is a view similar to FIGURE 4 but showing a still further modified embodiment of the invention;

FIGURE 7 is a view similar to FIGURE 6 but showing a further embodiment of the invention;

FIGURE 8 is an enlarged composite skirt end vieW of pistons of the prior art and the present invention for comparing the beam length effects of the skirts thereof;

FIGURE 9 is a skirt end view of a racing type engine piston with skirt walls at the inboard ends of the pin bosses and with narrow low friction but stiff cylinder wall engaging skirt portions; and

FIGURE 10 is a skirt end view of a conventional piston with skirt walls at the outboard ends of the pin bosses and extended flexible cylinder wall engaging areas.

As shown on the drawings:

The piston 10 of FIGURES 13 is a one-piece aluminum alloy piston formed either by casting, die forging or extrusion. Preferred aluminum alloys include eutectic and hyper-eutectic alloys of aluminum containing silicon and other alloying elements. The piston 10 has a circular head 11, a cylindrical ring belt or side wall 12 with three piston ring grooves 13 therein, a pair of diametrically opposed pin bosses 14 depending from the head 11 and having aligned wrist pin receiving apertures 15 adjacent their bottom ends, and an undulated skirt 16 depending from the ring belt or side wall 12 to a level below the pin bosses 14. The undulated skirt 16 has flat walls 17 at the inboard ends of the pin bosses .14 and cylindrical cylinder wall engaging portions 18 normal to the pin boss axis with bowed portions 19 connecting the flat areas 17 with the outer ends of the cylinder wall engaging portions 18. As illustrated in FIGURES 1 and 2, radial shoulders or faces 20 are provided under the ring belt or side wall 12 and the pin bosses 14 depend from these shoulders or faces to terminate at a level above the bottoms of the skirt 16. The skirt portions 18 which engage the cylinder walls are preferably longer than the flat sidewall portions 17 at the inboardends .of the pin bosses 14 and the bowed portions 19 slope from the short skirt wall portions 17 to the long skirt portions 18 as shown in FIGURE 3. A scalloped bottom is thus provided for the skirt, but if desired, the skirt could be of uniform height around its entire periphery.

The skirt 16 increases in thickness from the axial midpoints of the portions 18 to the flat faces 17. Thus as shown in FIGURE 2, the flat faces 17 are substantially 3 thicker than the portions 18 and these portions 18 thin down as they approach the mid areas thereof. The bulges or bowed portions 19 gradually increase in thickness as they approach the flats 17.

The piston 10a of FIGURES 4 and 5 differs from the piston in providing circumferential slots 21 through diametrically opposite sides of the ring belt or side wall 12 at the bottom of the lower ring groove 13. Parts identical with parts described in FIGURES 1-3 have been marked with the same reference numerals. It will be noted from FIGURE 5 that the side wall or ring belt 12 has slots 21 therethrough extending normal to the pin bosses 14. These slots 21 are longer than the connecting portions shown in section, which join the side wall 12 with the skirt. The slots 21 increase the flexibility of the skirt faces 18 and provide a greater hoop stretching action in operation than a non-slotted piston.

In the piston 10b of FIGURE 6, the slots such as 21 in the bottom groove of the ring belt area are provided as in the piston 10a and in addition, the cylinder wall engaging portions 18 of the piston skirt have vertical slots 22 intersecting the slots 21 and terminating in an enlarged circular end 23 at about the wrist pin axis for the piston. The slot 22 is at the axial center of the skirt portion 18. This piston 10b is known as a T-slot piston.

The piston 100 of FIGURE 7 is the same as the piston 10b except for the provision of inclined slots in the skirt portion 18. As shown, a pair of inclined slots 24 intersect the slot 21 and depend downwardly and outwardly adjacent the sides of the skirt portion 18. The slots 24 also terminate in enlarged rounded ends 25 but level with the wrist pin axis of the piston. This piston is known as a double U-slot piston and has enhanced skirt flexibility.

It should be understood that the pistons 10a, 10b and 10c are further examples of the undulated skirt piston 10 of FIGURES l-3 wherein skirt flexibility is increased by translots 21 either on one or both sides of the piston, with T-slots intersecting the translots 21 either on one side or both sides of the piston and with double U- slots either on one or both sides of the piston.

FIGURE 8 is an enlarged exaggerated skirt end view comparing the pistons of this invention with the prior art pistons shown in FIGURES 9 and 10. The piston of FIGURE 9 is exemplary of the so-called stifi racing pistons with a circular head 31 extending to flat radial shoulders 32, with pin bosses 33 depending from the head below the shoulders 32 and with a very narrow skirt 34 depending from the head having flat sides 35 at the inboard ends of the pin bosses 33 and very narrow cylinder wall engaging portions 36 between the flats 35. This piston 30 has very low operating friction due to the reduced skirt areas 36, but the straight flat side walls 35 produce a very rigid skirt and the portions 36 must have a very loose fit with the cylinder. The rigid and loose fit skirt produces noisy operation.

The piston 40 of FIGURE 10 is the other example of prior art pistons where the circular head 41 extends to small radial shoulders 42 and where the skirt is at the outboard ends of the pin bosses 43. As shown the skirt 44 has flat sides 45 at the outboard ends of the pin bosses 43 and has cylindrical cylinder wall engaging portions 46 extending almost completely around the circumference of the piston. This piston 40 is quite flexible, but because of its extensive cylinder wall engaging skirt portions 46, it has a high friction operating characteristic.

As shown in FIGURE 8, the pistons 10 of this invention are compared with the prior art pistons 30 and 40 by superimposing corresponding skirt portions thereon. The

skirt 16 of the pistons of this invention is illustrated in section, the skirt 34 of the racing piston 30 is illustrated in dotted lines and the skirt 44 of the prior art piston 40 is illustrated in solid lines. Common pin bosses 14 have been illustrated with the skirt 44 having its flats 45 at the outboard ends of these pin bosses while the flats 17 and 35 of the pistons 10 and 30 are at the inboard ends of the pin bosses. The cylinder wall engaging portions 18 of the skirt 16, 36 of the skirt 34, and 46 of the skirt 44, in operation, are all subjected to side loads normal to the wrist pin axis. In FIGURE 8, force vectors along the plane A, A designated by the arrows illustrate the side load on the piston. Force vectors along the plane E, 'E, designated by the arrows, represent the resultant of the thermal gradients in the piston due to heat expansion. Both pairs of forces A, A and E, E tend to collapse the piston skirts in the direction of the arrows A, A. In the case of slotted pistons, illustrated in FIGURES 4-7, the result of these outward forces in plane E, E pulls the skirt in the direction shown by the arrows in plane A, A by means of a hoop stretching action. This counteracts piston expansion. This size control, coupled with a flexible skirt, produces a quiet piston.

In the narrow skirt piston 30, the cylinder wall engaging skirt portions 36, lie along the plane BB' as shown in FIGURE 8. The load F-applied to the skirt 36 is, therefore, supported by a beam length AB. The supporting beam length of the full skirt 44, is illustrated at AD. Thelonger beam length AD thus provides a more flexible skirt than the short beam length AB. However, as indicated above, the full skirt flexible piston has the disadvantage of adding friction due to the extensive contact between the skirt and the cylinder wall.

The pistons 10 of this invention, on the other hand, have the undulated skirt 16 bowed outwardly from the narrow skirt 36 of the racing piston 30, but only to approximately the longitudinal midpoint of the pin boss 14 designated in the plane C, C. The bowed extremity of the undulated skirt then turns inwardly back to the fiat 17 at the inboard end of the pin boss 14. Therefore, the supporting beam length for the skirt 16 is (AB)+(BC)+(CB). This longer beam length approaches the length of the full skirt 46, but cylinder wall contact is terminated just beyond the area of contact of the very narrow racing piston skirt 36. As a result, a much more flexible skirt is provided without adding cylinder engagement area to the skirt.

In the illustrated embodiment, the maximum width of 'the cylinder wall engaging skirt portions 18 of the piston 10 lies between the midpoints of the pin bosses 14 and the beam length of the undulated skirt is the same as that of the conventional piston with skirt walls extending to the outboard ends of the pin bosses. This is true because of the beam length BC equal CD and the conventional piston has a beam length of (AB)+(BC)+(CD).

For maximum weight conservation consistent with load on the piston and the elastic limit of the material, the thickness of the skirt 16 varies from a maximum thickness at the flats or side walls 17 to the thinnest points at the centers of the thrust faces 18. A difference in thickness of .04 and .08 inches produces a very desirable flexible skirt.

Pistons of this invention are readily formed in dies without the die stresses encountering formation of the stiff narrow skirt pistons 30. In the formation of the pistons 30, the side walls 35 of the skirt require mow'ng of metal away from the die whereas in the undulated skirt arrangement of this invention, mass is added back into the punch and die thereby making possible the production of stronger forming apparatus and making it easier to cool such apparatus.

As indicated, the pistons of this invention are particularly adapted for manufacture by extrusion. The extruded aluminum material has a higher elongation than cast aluminum. Normal aluminum castings only have an elongation of 1% to l /2% while the same alloy material, formed by the extrusion process, exhibits an elongation of 6% to 8% at room temperatures. At elevated temperatures, these advantages are more pronounced since at 600 F. the cast piston elongation is only 5% to 6%,

but when extruded, it becomes 55%. The higher elongation makes it possible to design an extruded .piston with minimum sections for maximum flexibility without risk of piston failure when the elastic limit of the piston is exceeded under emergency operating conditions.

From the above descriptions, it will, therefore, be understood that this invention now provides a quiet operating, strong, light weight, easily manufactured engine piston with a flexible skirt having limited cylinder wall contacting areas to enhance low friction operation.

Although minor modifications might be suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent granted hereon, all such modifications as reasonably and properly come within the scope of my contribution to the art.

I claim as my invention:

1. An engine piston comprising a metal body having a head with a ring belt area, a plurality of piston ring grooves in the ring belt area, a skirt depending from the head, diametrically opposed pin bosses depending from the head, said skirt having opposed cylinder wall contacting areas normal to the pin boss axis and side wall areas inwardly from the outboard ends of the pin bosses together with curved areas joining the cylinder wall engaging areas and side wall areas which are bowed outwardly from the side wall areas to enhance the supporting beam length of the cylinder wall engaging areas.

2. An engine piston which comprises the head, said head having ring grooves around the side thereof and terminating in radial shoulders just below the ring grooves, diametrically opposed pin bosses depending from said head below said shoulders, an undulated skirt depending from said head having sides inwardly from the outboard ends of the pin bosses and cylinder wall engaging areas normal to the pin boss axis together with curved portions joining the sides and cylinder wall areas which bulge outwardly from the side walls and curve inwardly to the cylinder wall areas to add supporting beam length thereto without increasing the cylinder contact area thereof.

3. A piston which comprises a metal body having a piston head, piston ring grooves around the head, opposed laterally extending pin bosses below the head, a skirt depending from the head, and said skirt having an undulating contour with cylinder wall engaging areas normal to the pin boss axis and side walls inwardly from the outboard ends of the pin bosses together with outwardly bulged connecting portions adding supporting beam length to the cylinder wall portions without increasing the cylinder wall contact areas thereof.

4. A metal engine piston which comprises a die forged aluminum body having a cylindrical head, diametrically opposed pin bosses depending from the head, ring grooves around the head, and an undulating skirt depending from the head with diametrically opposed cylinder wall engaging areas normal to the pin boss axis, side walls at the inboard ends of the pin bosses, and bulged connecting portions between the side walls and cylinder wall engaging portions adding supporting beam length to the cylinder wall engaging portions without increasing the cylinder engaging areas thereof.

5. In an engine piston, the improvement of an undulating skirt having opposed cylinder wall engaging portions and flat side walls radially inward from the cylinder wall engaging portions and connected thereto through outwardly bulged portions.

6. In an engine piston, the improvement of a flexible skirt having limited cylinder wall engaging areas and flat side walls joined to said areas through outwardly bulged portions.

7. An engine piston which comprises a metal body having a head, diametrically opposed laterally extending pin bosses, and a skirt, said skirt having cylinder wall engaging areas depending from the head normal to the Wrist pin axis of the piston and flats at the inboard ends of the pin bosses together with bulged areas joining the cylinder wall engaging areas and the flats having outer extremities in a plane extending through the longitudinal midpoints of the pin bosses.

8. An engine piston comprising a head, diametrically opposed pin bosses depending from the head, a skirt depending from the head, and said skirt having an undulated contour with flat side walls at the inboard ends of the pin bosses, cylinder wall engaging portions normal to the pin boss axis and outwardly bulged connecting portions between the side walls and cylinder wall engaging portions, and said skirt decreasing in thickness from the flat side walls to the midpoints of the cylinder wall engaging portions.

No references cited.

MARTIN P. SCHWADRON, Primary Examiner.

R. R. BUNEVICH, Assistant Examiner. 

5. IN AN ENGINE PISTON, THE IMPROVEMENT OF AN UNDULATING SKIRT HAVING OPPOSED CYLINDER WALL ENGAGING PORTIONS AND FLAT SIDE WALLS RADIALLY INWARD FROM THE CYLINDER WALL ENGAGING PORTIONS AND CONNECTED THERETO THROUGH OUTWARDLY BULGED PORTIONS. 